Relocation (= move+destroy)

[official-gcc.git] / gcc / doc / ux.texi

@c Copyright (C) 2018 Free Software Foundation, Inc.
@c Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node User Experience Guidelines
@chapter User Experience Guidelines
@cindex user experience guidelines
@cindex guidelines, user experience

To borrow a slogan from
 @uref{https://elm-lang.org/blog/compilers-as-assistants, Elm},

@quotation
@strong{Compilers should be assistants, not adversaries.}  A compiler should
not just detect bugs, it should then help you understand why there is a bug.
It should not berate you in a robot voice, it should give you specific hints
that help you write better code. Ultimately, a compiler should make
programming faster and more fun!
@author Evan Czaplicki
@end quotation

This chapter provides guidelines on how to implement diagnostics and
command-line options in ways that we hope achieve the above ideal.

@menu
* Guidelines for Diagnostics::       How to implement diagnostics.
* Guidelines for Options::           Guidelines for command-line options.
@end menu


@node Guidelines for Diagnostics
@section Guidelines for Diagnostics
@cindex guidelines for diagnostics
@cindex diagnostics, guidelines for

@subsection Talk in terms of the user's code

Diagnostics should be worded in terms of the user's source code, and the
source language, rather than GCC's own implementation details.

@subsection Diagnostics are actionable
@cindex diagnostics, actionable

A good diagnostic is @dfn{actionable}: it should assist the user in
taking action.

Consider what an end user will want to do when encountering a diagnostic.

Given an error, an end user will think: ``How do I fix this?''

Given a warning, an end user will think:

@itemize @bullet
@item
``Is this a real problem?''
@item
``Do I care?''
@item
if they decide it's genuine: ``How do I fix this?''
@end itemize

A good diagnostic provides pertinent information to allow the user to
easily answer the above questions.

@subsection The user's attention is important

A perfect compiler would issue a warning on every aspect of the user's
source code that ought to be fixed, and issue no other warnings.
Naturally, this ideal is impossible to achieve.

@cindex signal-to-noise ratio (metaphorical usage for diagnostics)
@cindex diagnostics, false positive
@cindex diagnostics, true positive
@cindex false positive
@cindex true positive

Warnings should have a good @dfn{signal-to-noise ratio}: we should have few
@dfn{false positives} (falsely issuing a warning when no warning is
warranted) and few @dfn{false negatives} (failing to issue a warning when
one @emph{is} justified).

Note that a false positive can mean, in practice, a warning that the
user doesn't agree with.  Ideally a diagnostic should contain enough
information to allow the user to make an informed choice about whether
they should care (and how to fix it), but a balance must be drawn against
overloading the user with irrelevant data.

@subsection Precision of Wording

Provide the user with details that allow them to identify what the
problem is.  For example, the vaguely-worded message:

@smallexample
demo.c:1:1: warning: 'noinline' attribute ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      | ^~~
@end smallexample

@noindent
doesn't tell the user why the attribute was ignored, or what kind of
entity the compiler thought the attribute was being applied to (the
source location for the diagnostic is also poor;
@pxref{input_location_example,,discussion of @code{input_location}}).
A better message would be:

@smallexample
demo.c:1:24: warning: attribute 'noinline' on variable 'foo' was
   ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      |     ~~~ ~~~~~~~~~~~~~~~^~~~~~~~~
demo.c:1:24: note: attribute 'noinline' is only applicable to functions
@end smallexample

@noindent
which spells out the missing information (and fixes the location
information, as discussed below).

The above example uses a note to avoid a combinatorial explosion of possible
messages.

@subsection Try the diagnostic on real-world code

It's worth testing a new warning on many instances of real-world code,
written by different people, and seeing what it complains about, and
what it doesn't complain about.

This may suggest heuristics that silence common false positives.

It may also suggest ways to improve the precision of the message.

@subsection Make mismatches clear

Many diagnostics relate to a mismatch between two different places in the
user's source code.  Examples include:
@itemize @bullet
  @item
  a type mismatch, where the type at a usage site does not match the type
  at a declaration

  @item
  the argument count at a call site does not match the parameter count
  at the declaration

  @item
  something is erroneously duplicated (e.g. an error, due to breaking a
  uniqueness requirement, or a warning, if it's suggestive of a bug)

  @item
  an ``opened'' syntactic construct (such as an open-parenthesis) is not
  closed

  @c TODO: more examples?
@end itemize

In each case, the diagnostic should indicate @strong{both} pertinent
locations (so that the user can easily see the problem and how to fix it).

The standard way to do this is with a note (via @code{inform}).  For
example:

@smallexample
  auto_diagnostic_group d;
  if (warning_at (loc, OPT_Wduplicated_cond,
                  "duplicated %<if%> condition"))
    inform (EXPR_LOCATION (t), "previously used here");
@end smallexample

@noindent
which leads to:

@smallexample
demo.c: In function 'test':
demo.c:5:17: warning: duplicated 'if' condition [-Wduplicated-cond]
    5 |   else if (flag > 3)
      |            ~~~~~^~~
demo.c:3:12: note: previously used here
    3 |   if (flag > 3)
      |       ~~~~~^~~
@end smallexample

@noindent
The @code{inform} call should be guarded by the return value from the
@code{warning_at} call so that the note isn't emitted when the warning
is suppressed.

For cases involving punctuation where the locations might be near
each other, they can be conditionally consolidated via
@code{gcc_rich_location::add_location_if_nearby}:

@smallexample
    auto_diagnostic_group d;
    gcc_rich_location richloc (primary_loc);
    bool added secondary = richloc.add_location_if_nearby (secondary_loc);
    error_at (&richloc, "main message");
    if (!added secondary)
      inform (secondary_loc, "message for secondary");
@end smallexample

@noindent
This will emit either one diagnostic with two locations:
@smallexample
  demo.c:42:10: error: main message
    (foo)
    ~   ^
@end smallexample

@noindent
or two diagnostics:

@smallexample
  demo.c:42:4: error: main message
    foo)
       ^
  demo.c:40:2: note: message for secondary
    (
    ^
@end smallexample

@subsection Location Information
@cindex diagnostics, locations
@cindex location information
@cindex source code, location information
@cindex caret

GCC's @code{location_t} type can support both ordinary locations,
and locations relating to a macro expansion.

As of GCC 6, ordinary locations changed from supporting just a
point in the user's source code to supporting three points: the
@dfn{caret} location, plus a start and a finish:

@smallexample
      a = foo && bar;
          ~~~~^~~~~~
          |   |    |
          |   |    finish
          |   caret
          start
@end smallexample

Tokens coming out of libcpp have locations of the form @code{caret == start},
such as for @code{foo} here:

@smallexample
      a = foo && bar;
          ^~~
          | |
          | finish
          caret == start
@end smallexample

Compound expressions should be reported using the location of the
expression as a whole, rather than just of one token within it.

For example, in @code{-Wformat}, rather than underlining just the first
token of a bad argument:

@smallexample
   printf("hello %i %s", (long)0, "world");
                 ~^      ~
                 %li
@end smallexample

@noindent
the whole of the expression should be underlined, so that the user can
easily identify what is being referred to:

@smallexample
   printf("hello %i %s", (long)0, "world");
                 ~^      ~~~~~~~
                 %li
@end smallexample

@c this was r251239

Avoid using the @code{input_location} global, and the diagnostic functions
that implicitly use it---use @code{error_at} and @code{warning_at} rather
than @code{error} and @code{warning}, and provide the most appropriate
@code{location_t} value available at that phase of the compilation.  It's
possible to supply secondary @code{location_t} values via
@code{rich_location}.

@noindent
@anchor{input_location_example}
For example, in the example of imprecise wording above, generating the
diagnostic using @code{warning}:

@smallexample
  // BAD: implicitly uses @code{input_location}
  warning (OPT_Wattributes, "%qE attribute ignored", name);
@end smallexample

@noindent
leads to:

@smallexample
// BAD: uses @code{input_location}
demo.c:1:1: warning: 'noinline' attribute ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      | ^~~
@end smallexample

@noindent
which thus happened to use the location of the @code{int} token, rather
than that of the attribute.  Using @code{warning_at} with the location of
the attribute, providing the location of the declaration in question
as a secondary location, and adding a note:

@smallexample
  auto_diagnostic_group d;
  gcc_rich_location richloc (attrib_loc);
  richloc.add_range (decl_loc);
  if (warning_at (OPT_Wattributes, &richloc,
                  "attribute %qE on variable %qE was ignored", name))
    inform (attrib_loc, "attribute %qE is only applicable to functions");
@end smallexample

@noindent
would lead to:

@smallexample
// OK: use location of attribute, with a secondary location
demo.c:1:24: warning: attribute 'noinline' on variable 'foo' was
   ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      |     ~~~ ~~~~~~~~~~~~~~~^~~~~~~~~
demo.c:1:24: note: attribute 'noinline' is only applicable to functions
@end smallexample

@c TODO labelling of ranges

@subsection Coding Conventions

See the @uref{https://gcc.gnu.org/codingconventions.html#Diagnostics,
diagnostics section} of the GCC coding conventions.

In the C++ front end, when comparing two types in a message, use @samp{%H}
and @samp{%I} rather than @samp{%T}, as this allows the diagnostics
subsystem to highlight differences between template-based types.
For example, rather than using @samp{%qT}:

@smallexample
  // BAD: a pair of %qT used in C++ front end for type comparison
  error_at (loc, "could not convert %qE from %qT to %qT", expr,
            TREE_TYPE (expr), type);
@end smallexample

@noindent
which could lead to:

@smallexample
error: could not convert 'map<int, double>()' from 'map<int,double>'
   to 'map<int,int>'
@end smallexample

@noindent
using @samp{%H} and @samp{%I} (via @samp{%qH} and @samp{%qI}):

@smallexample
  // OK: compare types in C++ front end via %qH and %qI
  error_at (loc, "could not convert %qE from %qH to %qI", expr,
            TREE_TYPE (expr), type);
@end smallexample

@noindent
allows the above output to be simplified to:

@smallexample
error: could not convert 'map<int, double>()' from 'map<[...],double>'
   to 'map<[...],int>'
@end smallexample

@noindent
where the @code{double} and @code{int} are colorized to highlight them.

@c %H and %I were added in r248698.

Use @code{auto_diagnostic_group} when issuing multiple related
diagnostics (seen in various examples on this page).  This informs the
diagnostic subsystem that all diagnostics issued within the lifetime
of the @code{auto_diagnostic_group} are related.  (Currently it doesn't
do anything with this information, but we may implement that in the
future).

@subsection Spelling and Terminology

See the @uref{https://gcc.gnu.org/codingconventions.html#Spelling
Spelling, terminology and markup} section of the GCC coding conventions.

@subsection Fix-it hints
@cindex fix-it hints
@cindex diagnostics guidelines, fix-it hints

GCC's diagnostic subsystem can emit @dfn{fix-it hints}: small suggested
edits to the user's source code.

They are printed by default underneath the code in question.  They
can also be viewed via @option{-fdiagnostics-generate-patch} and
@option{-fdiagnostics-parseable-fixits}.  With the latter, an IDE
ought to be able to offer to automatically apply the suggested fix.

Fix-it hints can be added to a diagnostic by using a @code{rich_location}
rather than a @code{location_t} - the fix-it hints are added to the
@code{rich_location} using one of the various @code{add_fixit} member
functions of @code{rich_location}.  They are documented with
@code{rich_location} in @file{libcpp/line-map.h}.
It's easiest to use the @code{gcc_rich_location} subclass of
@code{rich_location} found in @file{gcc-rich-location.h}, as this
implicitly supplies the @code{line_table} variable.

For example:

@smallexample
   if (const char *suggestion = hint.suggestion ())
     @{
       gcc_rich_location richloc (location);
       richloc.add_fixit_replace (suggestion);
       error_at (&richloc,
                 "%qE does not name a type; did you mean %qs?",
                 id, suggestion);
     @}
@end smallexample

@noindent
which can lead to:

@smallexample
spellcheck-typenames.C:73:1: error: 'singed' does not name a type; did
   you mean 'signed'?
   73 | singed char ch;
      | ^~~~~~
      | signed
@end smallexample

Non-trivial edits can be built up by adding multiple fix-it hints to one
@code{rich_location}.  It's best to express the edits in terms of the
locations of individual tokens.  Various handy functions for adding
fix-it hints for idiomatic C and C++ can be seen in
@file{gcc-rich-location.h}.

@subsubsection Fix-it hints should work

When implementing a fix-it hint, please verify that the suggested edit
leads to fixed, compilable code.  (Unfortunately, this currently must be
done by hand using @option{-fdiagnostics-generate-patch}.  It would be
good to have an automated way of verifying that fix-it hints actually fix
the code).

For example, a ``gotcha'' here is to forget to add a space when adding a
missing reserved word.  Consider a C++ fix-it hint that adds
@code{typename} in front of a template declaration.  A naive way to
implement this might be:

@smallexample
gcc_rich_location richloc (loc);
// BAD: insertion is missing a trailing space
richloc.add_fixit_insert_before ("typename");
error_at (&richloc, "need %<typename%> before %<%T::%E%> because "
                     "%qT is a dependent scope",
                     parser->scope, id, parser->scope);
@end smallexample

@noindent
When applied to the code, this might lead to:

@smallexample
T::type x;
@end smallexample

@noindent
being ``corrected'' to:

@smallexample
typenameT::type x;
@end smallexample

@noindent
In this case, the correct thing to do is to add a trailing space after
@code{typename}:

@smallexample
gcc_rich_location richloc (loc);
// OK: note that here we have a trailing space
richloc.add_fixit_insert_before ("typename ");
error_at (&richloc, "need %<typename%> before %<%T::%E%> because "
                     "%qT is a dependent scope",
                     parser->scope, id, parser->scope);
@end smallexample

@noindent
leading to this corrected code:

@smallexample
typename T::type x;
@end smallexample

@subsubsection Express deletion in terms of deletion, not replacement

It's best to express deletion suggestions in terms of deletion fix-it
hints, rather than replacement fix-it hints.  For example, consider this:

@smallexample
    auto_diagnostic_group d;
    gcc_rich_location richloc (location_of (retval));
    tree name = DECL_NAME (arg);
    richloc.add_fixit_replace (IDENTIFIER_POINTER (name));
    warning_at (&richloc, OPT_Wredundant_move,
                "redundant move in return statement");
@end smallexample

@noindent
which is intended to e.g. replace a @code{std::move} with the underlying
value:

@smallexample
   return std::move (retval);
          ~~~~~~~~~~^~~~~~~~
          retval
@end smallexample

@noindent
where the change has been expressed as replacement, replacing
with the name of the declaration.
This works for simple cases, but consider this case:

@smallexample
#ifdef SOME_CONFIG_FLAG
# define CONFIGURY_GLOBAL global_a
#else
# define CONFIGURY_GLOBAL global_b
#endif

int fn ()
@{
  return std::move (CONFIGURY_GLOBAL /* some comment */);
@}
@end smallexample

@noindent
The above implementation erroneously strips out the macro and the
comment in the fix-it hint:

@smallexample
   return std::move (CONFIGURY_GLOBAL /* some comment */);
          ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          global_a
@end smallexample

@noindent
and thus this resulting code:

@smallexample
   return global_a;
@end smallexample

@noindent
It's better to do deletions in terms of deletions; deleting the
@code{std::move (} and the trailing close-paren, leading to
this:

@smallexample
   return std::move (CONFIGURY_GLOBAL /* some comment */);
          ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          CONFIGURY_GLOBAL /* some comment */
@end smallexample

@noindent
and thus this result:

@smallexample
   return CONFIGURY_GLOBAL /* some comment */;
@end smallexample

@noindent
Unfortunately, the pertinent @code{location_t} values are not always
available.

@c the above was https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01474.html

@subsubsection Multiple suggestions

In the rare cases where you need to suggest more than one mutually
exclusive solution to a problem, this can be done by emitting
multiple notes and calling
@code{rich_location::fixits_cannot_be_auto_applied} on each note's
@code{rich_location}.  If this is called, then the fix-it hints in
the @code{rich_location} will be printed, but will not be added to
generated patches.


@node Guidelines for Options
@section Guidelines for Options
@cindex command-line options, guidelines for
@cindex options, guidelines for
@cindex guidelines for options

@c TODO